When photochromic compounds are subjected to irradiation containing ultraviolet rays (sunlight, xenon lamps or mercury lamps), they undergo a reversible color change. As soon as the excitation stops, they regain their original color.
In recent years, organic materials intended for optical applications have been the subject of considerable research. Ophthalmic glasses, glass for the construction industry, motor vehicle or airplane windshields and helmet visors, whose transparency in the visible range can be modified by using photochromic compounds, have particularly attracted attention. For this type of application using sunlight (heliochromism), the photochromic active compound must satisfy a certain number of criteria, among which are:
high colorability in the visible range after excitation with light (colorability is a measure of the capacity of a photochromic compound to exhibit an intense color); PA1 an absence of coloration (or a weak coloration) in the initial state; PA1 rapid kinetics of thermal decolozization at room temperature; PA1 a high speed of coloration. PA1 in which Y is an acetylenic group directly linked to one of the 7 to 10 positions of the naphthopyran ring structure and R.sup.1 and R.sup.2 are, independently from each other, H, alkyl, cycloalkyl, aryl or heteroaryl. PA1 in which R is H, alkyl, cycloalkyl, aryl or heteroaryl. PA1 in which Z is H or an alkyl group, preferably a C.sub.1 -C.sub.6 alkyl group such as methyl, ethyl. PA1 In which R.sup.3 denotes H or a substituent which can occupy one of the position 2, 3, 3', 4, 4' and 5'. Preferably, R.sup.3 as a substituent, occupies position 2 or 5'. PA1 In which R.sup.a represents H, an alkyl group, an aryl group, a group --OR", --SR", --COR" and COOR", in which R" is defined as R' or is an amino group of formula NR.sup.4 R.sup.5 in which R.sup.4 and R.sup.5 represent, independently from each other, an alkyl group, a cycloalkyl group, an aryl group, R.sup.4 and R.sup.5 being capable to form with the nitrogen carbon atom, an aromatic heterocycle having 4 to 7 links and optionally including one or more heteroatoms selected from oxygen, nitrogen and sulfur, and n is an integer from 1 to 5. PA1 X=1 or Br PA1 n=0 or 1 PA1 Catalysts=Tetrakis[trisphenyl phosphine]palladium (Pd(PPh.sub.3).sub.4) and Copper (I) iodide (CuI) PA1 Solvent=Benzene PA1 Phase transfer agent=Benzyl triethyl ammonium bromide (C.sub.6 H.sub.5 CH.sub.2 N+(C.sub.2 H.sub.5).sub.3 Br.sup.-). ##STR11## PA1 NBS=N-Bromosuccinimide PA1 DMF=Dimethyl formamide PA1 Dppp=[1,3bis(diphenylphosphino)propane]dichloronickel(II) PA1 In which R.sub.5, R.sup.6, R.sup.7 and R.sup.8 identical or different represent the same groups as defined above for R.sup.1 and R.sup.2, and PA1 In which R.sup.3 is defined as above and can occupy one of the positions 3, 3', 4, 4'. PA1 with n being 0 or 1.
One of the major difficulties encountered with photochromic compounds is that of obtaining a compromise between high colorability and rapid decolorization kinetics. The reason for this is that, under continuous solar irradiation, a photostationary equilibrium is established between the molecules which become colored under the action of ultraviolet light and those which become decolorized under the conjugate action of temperature and visible light. Thus, frequently, an increase in the rate of decolorization entails a decrease in the colorability.
U.S. patent application Ser. No. 09/349,686 to Michel Frigoli et al, filed on Jul. 8, 1999 discloses a novel family of naphtho[2,1-b]pyrans substituted with bithienyl or terthienyl groups, which have particularly advantageous photochromic properties.
However, these novel naphtho[2,1-b]pyrans substituted with bithienyl or terthienyl groups, as well as the classical photochromic compounds of the naphtho[2,1-b]pyrans family, suffer of a lack of resistance to fatigue (also known as photodegradation of the photochromic compound).
In other words, the initial maximum colorability (Ao) of a photochromic compound decreases in time upon irradiation by visible light, and the resistance to fatigue of a photochromic compound is usually determined by measuring the time t for the colorability of the photochromic to decrease to a value equals to Ao/2 upon continuous irradiation by visible light. The longer is this time t the higher is the resistance to fatigue of the photochromic compound.
Thus, one object of the invention is to provide photochromic compounds having photochromic properties at least equivalent to those of the known photochromic compounds, in particular of the naphtho[2,1-b]pyrans family, but exhibiting significantly improved resistance to fatigue.
This combination of characteristics makes these novel compounds particularly advantageous for the manufacture of photochromic materials, in particular comprising a substrate made of transparent polymer material such as organic glasses with variable optical transmission (glass for sunglasses, glass for the construction industry, motor vehicle or airplane windshields, and riding or flying helmet visors).
The photochromic compounds can be incorporated directly into the organic glass substrate or dissolved in a polymer film stuck to the organic glass substrate.